The lack of a planet at the asteroid belt is due to the gravitational pull of Jupiter. The gravitational pull from Jupiter is so strong that asteroids are pulled away from their orbit, creating gaps in the belt called the Kirkwood gaps.
Early in the life cycle of the solar system, there was enough material in the main portion of the asteroid belt to create a planet four times the size of Earth. The presence of Jupiter, however, was enough of a destabilizing effect that these asteroids never coalesced into a single body. Instead, the asteroids grouped together in clusters of various sizes.
As of 2014, asteroids that come into resonance with the orbit of Jupiter still can be destabilized from their own orbit. The total mass of asteroids that are left in the main portion of the asteroid belt measures below 50 percent of the moon's mass.
Asteroids found in the belt typically have different chemical compositions. This difference suggests that the asteroids formed at different areas of the belt and can be grouped by multiple common origins. The size of asteroids in the belt ranges from small pebbles to the size of the Ceres asteroid, which is 950 kilometers in diameter.